The present invention relates generally to a multi-spindle unit attached to a spindle of a machine tool for supporting a plurality of cutting tools rotated by the machine spindle. More particularly, the invention is concerned with such a unit for rotary cutting tools, which allows the rotary cutting tools to perform high-precision machining, unaffected by positioning error of the unit with respect to the machine spindle.
Various tool holders have been used for mounting drills, milling cutters, reamers, boring bars and other rotary cutting tools on spindles of machine tools such as drilling, milling and boring machines, and machining centers which are capable of automatically performing multiple kinds of cutting operations. Such a tool holder supports a tool at its one end portion, and is adapted to be removably fixed to the machine spindle at the other end portion.
However, surfaces of the tool holder for fitting, positioning and other purposes with respect to the machine spindle are liable to wear due to mounting and dismounting of the tool holder to and from the machine spindle. Further, these surfaces are subject to cutting chips, dust and dirt, and other foreign matter. As a result, the tool holder does not always make a sufficiently snug fit in the spindle. Thus, it has been difficult or impossible to avoid a positioning error of the tool holder due to misalignment or inclination of its centerline with respect to the centerline of the spindle. This positioning error or misalignment of the tool holder results in an increase in run-out of the boring bar at the free end of the holder, and consequently leads to machining errors such as an oversize bore diameters bored by the boring bar. That is, the positioning error of the tool holder lowers the accuracy of machining with the tool. Such positioning error of a tool holder will give rise to serious trouble, particularly in fine-boring, reaming and other machining operations which require relatively high accuracy.
In view of the above problem, one of the inventors of the present application and his co-workers proposed, in the pending patent application Ser. No. 625,960 filed June 29, 1984 (assigned to the assignees of the present application), a holder for a rotary cutting tool which is capable of preventing a decrease in accuracy of machining by the tool due to positioning error of the holder with respect to the machine spindle. This tool holder has a holder body fixed to a spindle of a machine tool and a rotatable shaft disposed concentrically with the holder body and having a tool mounting portion at its one end. The rotatable shaft is coupled at its other end portion to the holder body for receiving a torque from the holder body and is axially and radially displaceable relative to the holder body. Further, the tool holder has a positioning member fixedly disposed radially outwardly of the spindle and a casing disposed radially outwardly of the rotatable shaft such that the casing and the shaft are rotatably relative to each other. The casing is engageable with the positioning member for accurate positioning thereof by the positioning member, and thereby positioning the rotatable shaft, while the holder body is fixed to the spindle.
In such a tool holder, a rotary cutting tool is fixed to one end portion of the rotatable shaft. The rotatable shaft is positioned by the casing, which is accurately positioned by the positioning member disposed fixedly around the outer circumference of the free end of the spindle. Accordingly, a possible misalignment of the holder body with respect to the spindle will not affect a machining accuracy of the rotaty cutting tool carried by the rotatable shaft. In other words, the accurate positioning of the casing permits a high-precision machining.
Thereafter, however, there have arisen requirements for improvements in not only machining accuracy but also machining efficiency, by accomplishing simultaneous machining of plural bores.